Templatised Soft Floating-Point for High-Level Synthesis

High-level Synthesis (HLS) tools have greatly increased the productivity of FPGA application development, making it possible to easily create highly-parallel application-accelerators. However, while FPGAs are known for the ability to customise the number representation of data-paths, most HLS work only uses custom-precision for fixed-point representations, and for floating-point relies on the 64-, 32-, and 16-bitformats provided by vendors. This paper presents a solution for parametrised floating-point in HLS via C++ templates, allowing for compile-time selection of exponent and fraction widths, including the use of mixed precisions for input arguments and result types. By using arbitrary width integers and compile-time logic the resulting operators describe the same data-path as an external floating-point IP generator, while still allowing the HLS tool to perform detailed optimisation and scheduling of the internal components. We show that the resulting custom-width HLS cores provide similar area and performance to platform-native vendor IP blocks, while adding full support for heterogeneous precision floating-point data-paths to HLS tools.